1. Field of the Invention
The present invention relates to a transducer utilizing surface acoustic waves (SAW), and more particularly to a surface acoustic wave transducer where internal reflections and electromechanical coupling constant changes are applied to an interdigital transducer (IDT).
2. Description of the Related Art
Surface acoustic wave transducers which excite and receive surface acoustic waves on a piezoelectric or electrostrictive material are widely used as devices such as bandpass filters. An ordinary surface acoustic wave transducer has electrodes of an IDT structure which are arranged on the surface of a piezoelectric or electrostrictive material. When an AC electric signal is applied between positive and negative electrodes of an IDT on the input side, a surface acoustic wave is excited, The excited surface acoustic wave is propagated to and received by another IDT on the output side. Such a surface acoustic wave transducer is known as a transversal surface acoustic wave filter.
The above surface acoustic wave transducer basically causes a loss of 6 dB because the excited surface acoustic wave is propagated equally in both left and right directions. Heretofore, various surface acoustic wave transducers have been proposed in the art in order to minimize the loss of 6 dB. The proposed surface acoustic wave transducers are generally classified into the following three types:
(a) A three-phase unidirectional device having three IDT electrode fingers (teeth) disposed on its surface, where signals with phase shifts of 0°, 120°, 240° are applied to the respective electrode fingers:
(b) A group-type unidirectional transducer having a meander line extending between ordinary electrode fingers and serving as a ground electrode, where signals having a phase shift of 90° are applied to the respective electrode fingers; and
(c) An internal-reflection-type unidirectional transducer having a pair of IDT electrode fingers made of aluminum (Al) and electrode fingers made of a large-density metal such as gold (Au) for reflecting surface acoustic waves, with the distance between a central region where surface acoustic waves are excited and a central region where surface acoustic waves are reflected being set to ⅛ of the wavelength of the excited surface acoustic waves.
In the following description, the wavelength of the excited surface waves is represented by λ0.
The three-phase unidirectional transducer at (a) can keep surface acoustic waves propagated in one direction in a wide frequency range. However, since the electrode fingers extending from one of the three bus bars need to extend over another one of the three bus bars, it requires a highly complex process and hence is highly costly to manufacture the three-phase unidirectional transducer. Another problem with the three-phase unidirectional transducer is that it needs a complex phase shifter.
The group-type unidirectional transducer at (b) needs a 90° phase shifter which specifically comprises a coil and also needs a meander line having a large total length, and causes a large filter insertion loss because of the ohmic loss of the long meander line.
The internal-reflection-type unidirectional transducer at (c) is a unidirectional interdigital transducer which requires no phase shifter with the exciting and reflecting positions shifted λ0/8 from each other, and is expected to provide excellent characteristics.
One internal-reflection-type unidirectional transducer which has heretofore been proposed has a double electrode structure including positive and negative electrodes spaced by an electrode gap of λ0/8 and divided into widths of λ0/8. After positive double electrodes or negative double electrodes are fabricated, a thin metal film is added to one of the double electrodes to provide a mass loading effect. The double electrodes have a structure in which each electrode finger of an IDT are divided into two segments, and are also called split electrodes. The internal-reflection-type unidirectional transducer is problematic in that it has a poor conversion efficiency because of the double electrodes employed, and is difficult to use as a high-frequency device as it has IDT electrodes having a spacing of λ0/8. Details of a mass-loading-effect unidirectional transducer are disclosed in, for example, C. S. Hartmann, et al., 1982 IEEE Ultrasonic Symposium Proceedings, pp. 40–45.
Another proposed internal-reflection-type unidirectional interdigital transducer having double electrodes is manufactured as follows: After one of double electrodes is formed on a substrate, a dielectric film having a uniform film thickness H0 is applied thereto, and then the other one of double electrodes is applied to the dielectric film, providing different electromechanical coupling constants. This internal-reflection-type unidirectional interdigital transducer operates based on the difference between the magnitudes of the electromechanical coupling constants. However, the proposed internal-reflection-type unidirectional interdigital transducer are disadvantageous in that the fabrication process is complicated, the conversion efficiency is poor because double electrodes are employed, and the transducer is difficult to use as a high-frequency device as it has an IDT having a spacing of λ0/8. An electromechanical-coupling-constant-changing-type unidirectional transducer is disclosed in, for example, K. Yamanouchi, et al., Electronic Letters, Vol. 20, No. 20, pp. 819–821, September 1982.
Apodized (weighted) transducers having IDT electrodes intersecting at varying intervals to achieve desired frequency characteristics have also been proposed in the art. However, the apodized transducers suffer problems in that electrodes for transmission and reception cannot be weighted and the apodized transducers have a weighing loss.